On 11 October 1995, Kuju Volcano (Central Kyushu, Japan) erupted steam and ash from new vents opened south of the active Kuju-Iwoyama fumarole area. The He/Ar atomic ratio of fumarolic gas, which is a possible indicator of the ratio of magmatic gas to air, suddenly decreased from 0.12-0.22 to 0.04-0.06 at the beginning of the 1995 eruption. The flux of magmatic He soon after the eruption is estimated to be almost 12 times larger than that before the eruption: values before and after the eruption are 57 x 10(-3) and 694 x 10(-3) kg/day, respectively. In contrast, the flux of air-derived He increased after the eruption by nearly 40 times than before the event: values before and soon after the eruption are 0.2 x 10(-3) and 7.6 x 10(-3) kg/day, respectively. The sudden decrease in the He/Ar ratio suggests that air has been suctioned into the flow path of uprising volcanic fluid through the surrounding formation of altered rocks piled loosely. The volume of air entrained is estimated to be nearly 1 x 10(4) m(3)/day. (C) 2000 Elsevier Science B.V. All rights reserved.